Transcript Bipolar Junction Transistor

unit-2
Bipolar Junction Transistor
[BJT’s]
S.Lakshmanan[ACED]
 Transistor is a solid state Device, whose operation is
based on flow of electric charge carriers with in the
solid.
 Used as Amplification and Switch
 It is analogous to a Vacuum triode.
 Transistor is a current controlling device.
 Vacuum triode is a Voltage controlled device.
 Transistor is only 5 decades old and replaces
vacuum triodes in all applications.
S.Lakshmanan[ACED]
compact size, light weight, rugged
construction, more resistive to shocks
and vibration, instantaneous
operation[ no heating required], low
operating voltage, high operating
efficiency[no heat loss] long life with
efficiency.
Draw back:- loud hum noise,
restricted operating temp and freq.
S.Lakshmanan[ACED]
structure
S.Lakshmanan[ACED]
Types and symbol
• NPN
• PNP
S.Lakshmanan[ACED]
 Transistor is like two PN-junction diodes
connected back to back.
 Each Transistor has two PN-junction onejunction b/w Emitter and Base {Emitter junction}
and other-junction b/w Base and
Collector{Collector junction}.
 These two junction give rise to three terminals
Emitter, Base , Collector.
 Transistor is single crystal in which there is twopn junction. Idea behind is one section to supply
the charges to be collected by the third section
through the middle section.
 Supply free charges is called Emitter, Collecting
these charges Collector, the middle section
formed b/w Emitter and Collector is Base.
S.Lakshmanan[ACED]
Terminals
Emitter:- the E is always forward biased w.r.t base
so that it can be supply majority charge carriers to
the base. The E is heavily doped so that it may be
able to inject a large number of charge carriers.
Collector:- the main function is to collect the
majority charge carriers. Collector is always reverse
biased so as to remove the charge carriers away
from its junction with the base. It is moderately
doped.
Base:- very lightly doped and very thin. It passes
most of the injected charge carriers to the collector.
Base forms two junctions i.e. emitter base and
collector base.
S.Lakshmanan[ACED]
 EB junction is F.B it offers low resistance to
the emitter current. The CB junction is R.B
offers high resistance to the collector current.
 The resistance of EB junction is very small as
compared to the CB junction, therefore the F.B
applied to the EB junction is very small
whereas the R.B on the CB junction is much
larger.
 Collector region is made physically larger than
Emitter region Bcoz collector is to dissipate
much power[heat]. No Arrow is marked for
collector, Since its leakage current is always
opposite to the direction of emitter current.
S.Lakshmanan[ACED]
Transistor Action
• Transistor NPN
PNP
Behaves exactly in the same way except
change in biasing and majority carriers.
PNP the conduction is by holes
NPN the conduction is by electrons
NPN is more used than PNP bcoz it has
better high frequency response & high
mobility.
S.Lakshmanan[ACED]
Unbiased Transistor
When no battery is connected b/w the
terminals of a transistor, the
transistor is said to be an unbiased
transistor or open circuit state.
Semiconductor is doped with donor
impurity or Pentavalent it become Ptype. If its doped with acceptor or
trivalent it becomes N-type
semiconductor.
S.Lakshmanan[ACED]
 When a N-region is formed next to P-region, a
barrier potential is produced at the PN-junction.
 The free electrons in the N-region diffuse into the
adjacent P-region to annihilate holes.
Consequently, a layer of positive ions is formed on
the N side, layer of Negative ions formed on the Pside. Creates the Barrier potential.
 In NPN transistor- the EB depletion region width is
smaller than the CB depletion region width.
S.Lakshmanan[ACED]
S.Lakshmanan[ACED]
Operation of Transistor
 The process of applying DC voltage across the
different terminals of a transistor[to check
whether the transistor is operating in the
active, cut off or the saturation region] is
called Biasing.
 Normal operation the EB junction is always
F.B while the CB junction is R.B
 F.B at the EB junction reduces the barrier
potential and narrows the depletion region.
 The lightly doped base and collector region
produce a wide depletion region under R.B
S.Lakshmanan[ACED]
 The electrons are injected into the E region by
the EB supply VEB.
 These conduction band e have enough energy to
overcome the EB barrier potential.
 The injected e enter into very thin, lightly doped
base region bcoz the base is very lightly doped
relative to the E region, only the few of the
electrons recombine with the holes doped into
the base.
 Injection of e makes the electron concentration
on the emitter junction very large and collector
junction the concentration of the e is extremely
small and large in base.
 Injected electron diffuse into collector region due
to extremely small thickness of base which is
much less than the diffusion length.
 Most of the electron cross into collector junction.
S.Lakshmanan[ACED]
 Collector is reverse biased and creates a strong
electrostatic field b/w base and collector.
 The field immediately collects the diffused
electrons which enter the collector junction.
 Flow of electrons into the base region when
confronted with hole, a few electrons [1 to 5%]
combine and neutralize, rest of the electrons[95 to
99%] of the injected electrons diffuse into the
collector region and collected by the collector
electrode.
 To maintain the base neutrality base electrode
provides equal No.oF electrons which have
combined with holes and results in a base
current.
 The emitter current is equal to the sum of
collector current and base current.
IE = IC + IB
S.Lakshmanan[ACED]
 The ratio of collector current to emitter
current is called α [alpha]
 The ration of collector current to base
current is called β [beta]
 In a transistor the α can not be greater than
unity.
 The FB tends to align the V.B and C.B while
the RB tends to increase their misalignment.
 Further, the electron lose energy in moving
from base region into collector region. Only a
few amount e is recombined with h in base
region.
S.Lakshmanan[ACED]
Why the C region is more
than E region?
• The potential hill b/w the base and
collector region is very steep.
• Specially, electron much loos energy
when they move from B to C. the
electron give up their energy in the
form of heat.
• The C region must be able
To dissipate this heat energy.
S.Lakshmanan[ACED]
S.Lakshmanan[ACED]
Configurations
• Common Emitter
• Common Base
• Common Collector
S.Lakshmanan[ACED]
Common Emitter Configuration
S.Lakshmanan[ACED]
Common Base Configuration
S.Lakshmanan[ACED]
CB Configuration
 In a common base configuration, the input current is the emitter
current. and the output current is the collector current I. The ratio
of change in collector current to the change in emitter current at
constant collector-base voltage is called current amplification
factor.
 A test circuit for determining the static characteristic of an NPN
transistor is shown. In this circuit, base is common to both the
input and the output circuits. To measure the emitter and the
collector currents mull ammeters are connected in series with the
emitter and the collector circuits.
 Voltmeters are connected across the input and the output circuits to
measure VBE and VCB .There are two potentiometers R1 and R2 to
vary the supply voltages VCC
and VBE.
S.Lakshmanan[ACED]
CB - i/p Characteristics
 It is a curve, which shows the relationship between the emitter
current, I and emitter-base voltage V at constant collector-base
voltage V This method of determining the characteristic is as
follows
 First by means of R1, a suitable voltage is applied to VCB from
VCC. Next, voltages VBE is increased in a number of steps and
corresponding values of IE are noted.
 The emitter current is taken on the Y-axis and the emitter base
voltage is taken on the X-axis as shows the input
characteristic for germanium and silicon transistors.
The following points may be noted from the characteristics
curves.
 This characteristic may be used to find the input resistance of
a transistor. The input resistance (ri) value is given by the
reciprocal of the input characteristic curve.
 The emitter current IE increases rapidly with small increase in
emitter- base voltage VBE. It means that the input resistance is
very high. The emitter current is dependent of collector voltage.
S.Lakshmanan[ACED]
o/p Characteristics
 It is a curve which shows the relationship between the collector
current IC and the collector-base voltage VCB at constant emitter
current IE. This method of determining the characteristic is as
follows.
 First, by means of R2 a suitable voltage is applied to the base and
the emitter. Next, VCB is increased from zero in a number of steps
and corresponding values of IC are noted.
 The above whole procedure is repeated for different values of IE for
obtaining family of curves.
 The collector-base voltage is taken the X-axis. shows the family of
output characteristics at different emitter current values.
 The following points may be noted from the family of
characteristic curves.
S.Lakshmanan[ACED]
 The collector current IC varies with VCB only at very
low voltages. This characteristic may be used to find
the output resistance (ro)
 A very large change in collector-base voltage produces
small change in collector current. It means that the
output resistance is very high.
 The collector current is constant above certain values
of collector-base voltage. It means that IC is
independent of VCB and depends upon IE only.
The output characteristics may be divided into three
regions
1. The active region
2. Cut-off region
3. Saturation region
S.Lakshmanan[ACED]
ACTIVE REGION
• In this region the collector junction is reverse
biased and the emitter junction is forward
biased. In this region when IE=0, IC =ICO. This
reverse saturation current remains constant
and is independent of collector voltage V as
long as is below the break down potential.
When emitter current flows in the emitter
circuit then a fraction (- IE) of this current
reaches the collector. Hence IC= -IE+ICO. Thus
in the active region the collector current is
independent of collector voltage and depends
only upon the emitter current. But due to
Early effect there is a small increase (0.5%) in
IC with increase in S.Lakshmanan[ACED]
VCB
Saturation Region
The region to the left of the ordinate VCB=0 is called
the saturation region. In this region both junctions
are forward biased. This is also called as bottomed
region because the voltage has a fallen near the
bottom of the characteristic where VCB=0. In this
region IC increases rapidly with even small increase
VCB in as shown in Fig.
Cut-off Region
The region below the IE=0 characteristic, for which
the emitter and collector junction are both reverse
biased, is called cut-off region. This portion of
characteristic is not coincident with the voltage axis
as shown in Fig.
S.Lakshmanan[ACED]
Region of Operation
Conditions
Emitter-Base
Junction [EB]
Collector-Base
Junction [CB]
Region Of
Operation
Forward-Reverse
Forward-Biased
Reverse-Biased
Active
Forward-Forward
Forward-Biased
Forward-Forward
Saturation
Reverse-Reverse
Reverse-Biased
Reverse-Biased
Cut-Off
Reverse-Forward
Reverse-Biased
Forward-Biased
Inverted
S.Lakshmanan[ACED]
 Forward-reverse biasing:- the transistor is in
active region and the collector current depends
upon the emitter current.
 The transistor is operated in this region for
amplification.
 Forward-Forward biasing:-in this, both collector
and emitter are forward biased, the transistor
operates in the saturation region, the collector
current becomes independent of the base current.
 Transistor acts as Closed switch.
 Reverse-Reverse biasing:- both the junction are
reverse biased. In this the transistor has practically
zero current because the emitter does not emit
charge carriers into the base and no charge
carriers are collected by the collector.
 Transistor acts as Open Switch.
S.Lakshmanan[ACED]
Bipolar Transistor
Configurations
Characteristic
Common
Base
Common
Emitter
Common
Collector
Input Impedance
Low
Medium
High
Output Impedance
Very High
High
Low
Phase Angle
0o
180o
0o
Voltage Gain
High
Medium
Low
Current Gain
Low
Medium
High
Power Gain
Low
Very High
Medium
S.Lakshmanan[ACED]
Transistor biasing
The most basic ways of Biasing are..
 Base-Current Bias (Fixed Bias)
 Self-Bias
 Combination Bias.
S.Lakshmanan[ACED]
Base-Current Bias (Fixed
Bias)
• In this bias method, a biasing resistor is connected between
the collector supply and the base. This is a very simple
arrangement but carries a heavy withdraw. It is very unstable
in means of temperature changes.
• If the temperature of the transistor changes, either by
changing of ambient temperature or from current flow within
the transistor, the dc operating point (AKA quiescent or static
point) will change.
• This change is most undesirable because it affects the
amplification gain and may also result into distortion on the
output signal
S.Lakshmanan[ACED]
Self-Bias
 In this type of biasing, the biasing resistor is placed
directly between the collector and the base of the
resistor.
 With this biasing, feedback voltage can be fed from
the collector to the base to develop forward bias.
 This is a better connection because if temperature
increases and cause an increase in collector
current, the collector voltage will fall because of the
increase of voltage produced across the load
resistor RL.
 This drop in VC will be fed back to the base and
will result in a decrease in the base current.
S.Lakshmanan[ACED]
 The decrease in base current will oppose the original
increase in collector current and tend to stabilize it.
 The exact opposite effect is produced when the
collector current decreases.
 The temperature change with this way will be
managed as long as it is a moderate ambient
temperature change.
 It cannot handle large temperature changes. Also,
Bcoz the signal on the collector affects the base
voltage and because they have 180 degrees phase
difference, the amplification is slightly reduced.
 This is also known as "negative feedback",
sometimes useful to prevent output signal distortion
and self bias may be used for this purpose.
S.Lakshmanan[ACED]
Combination Bias.
 It’s a combination of fixed and self bias.
 the stability of the operation of the transistor is
improved and also the disadvantages of the other
methods are overcome.
 The fixed bias is implemented using the voltage
divider R1-R2. The current flowing through the
voltage divider network biases the base positive
with respect to the emitter.
 the voltage divider tend to keep the base bias
steady while the emitter bias changes with
emitter conduction.
S.Lakshmanan[ACED]
 The self bias is implemented using the resistor R3
connected in series with the emitter. If IE increases,
the voltage drop across R3 will also increase and
thus the Vc will be reduced.
 A drawback is that again the amplification is
slightly reduced, but the stability is dramatically
increased.
 To provide further more thermal stability and allow
minimal signal degeneration, a bypass capacitor
Cbp is places parallel to R3.
 Cbp is large so rapid signal variations will not
change its charge materially and no degeneration
of the signal will occur.
S.Lakshmanan[ACED]
AMPLIFIER IMPEDANCE’s
S.Lakshmanan[ACED]
Transistor as Amplifier
S.Lakshmanan[ACED]
 the resistor R1, R2, and RE from the biasing
stabilization circuit.
 The biasing circuit must establish a proper operating
point otherwise a part of the –ve half cycle of the signal
may be cut-off in the output.
 RL is the load, it represent the i/p resistance for the
next stage.
 Cin is called i/p capacitor, used to couple the signal to
the transistor base.
 CE is the bypass capacitor used in parallel with emitter
resistance RE in order to provide a low reactance path
to the amplifier ac signal.
 In the absence of this capacitor, the amp ac signal
flowing through RE will cause a voltage drop across it,
which in turn will feedback the i/p side and reduce the
o/p voltage.
S.Lakshmanan[ACED]